Energy intake and dietary macronutrient composition is the most difficult part of the energy balance equation to measure. Using an in patient computerized vending machine system, food intake has been measured over 180 individuals. In individuals undergoing the study more than once the intra-class correlation coefficient is very high (r=0.9) indicating that these studies although performed in an inpatient setting are very reproducible. Higher respiratory quotient, the ratio of carbohydrate oxidation to lipid oxidation, is known to predict weight gain. The mechanism of this increase in weight was thought to be secondary to a lower lipid oxidation in these individuals. Higher respiratory quotient measured during weight maintenance also predicted food intake during the ad-libitum vending machine study. However the component of respiratory quotient which best predicted food intake was higher carbohydrate oxidation indicating a role for carbohydrate (likely glycogen) balance in the regulation of short term food intake. To further investigate whether carbohydrate balance influences food intake, we fed individuals (in random order) a high fat versus high carbohydrate diet followed by ad libitum food intake. We did not find any difference in food consumption after either diet. However, more obese individuals tended to eat more after the high fat compared with the high carbohydrate diet and the opposite was true in leaner individuals, indicating that adiposity is associated with alteration in responses to ingested macronutrients. We have also investigated the effects of overfeeding on energy intake and physical activity. In a random cross over design we found that individuals did not down regulate energy intake after 3 days of overfeeding by 150% of their weight maintaining requirements. We also did not find any significant changes in circulating gut or adipose tissue hormones with overfeeding. During the overfeeding period, we did find that weight gain was associated with an increase in sedentary behavior indicating either that increased energy intake may predispose some individuals to become more sedentary or that resistance to sedentary behavior may help moderate weight gain during times of overconsumption. To develop methods to measure free living food intake, we have ongoing studies which are validating chewing and swallowing monitors using our automated vending machines. We have completed a pilot study in which 10 volunteers wore these monitors, and are currently analyzing the data to determine if these monitors can identify episodes of food ingestion and whether identification and duration of these episodes is correlated with overall calorie intake. We have also begun a study which will investigate stable isotopes as biomarkers for dietary patterns. Individuals (n=40) will consume diets over 3 months which vary by meat, soda and fish content and plasma, hair, and adipose tissue samples will be analyzed for changes in the stable isotopes C13 and N15. We will also investigate how these dietary patterns affect behavioral and performance testing, glucose tolerance and energy expenditure. We have currently completed 8 volunteers in this intensive study which requires a 14 week stay on our inpatient unit.